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Posted: August 28, 2009

Smoothing the way to superior OLED displays

(Nanowerk News) Electrospray-deposited polymer films can be used to make organic light-emitting diodes (OLEDs) with better characteristics than those made from spin-coated films, according to Yutaka Yamagata of the RIKEN Center for Intellectual Property Strategies, Wako, and colleagues. These researchers have used a novel dual-solvent concept to make the electrospray-deposited films smoother than before, thereby enabling the superior devices to be built ("Thin-Film Fabrication Method for Organic Light-Emitting Diodes Using Electrospray Deposition").

OLED displays are strong competitors with LC screens like this one.

Organic light-emitting diodes are now entering the market place as screens for mobile phones and televisions (Fig. 1), and mass-production techniques are needed to simplify the manufacturing process and reduce costs and wastage.

Previous attempts to use the electrospray-deposition technique for OLED fabrication have failed to produce polymer films that compete with other fabrication techniques. Yamagata and colleagues decided to use a combination of two solvents to improve this technique, which uses a thin glass capillary with the polymer solution stored inside and a conductive wire inserted in it. When a high voltage is applied between this conductive wire and the OLED electrodes on the substrate, the solution sprays out of the capillary end as atomized droplets that are attracted to the substrate by electrostatic force. This means there is little solution wastage as the spray is highly directed.

They found that the first solvent evaporated rapidly after the atomization of the solution, leaving a small amount of the second solvent, which has a higher boiling point, in the droplets. When the polymer concentrations were finely tuned, the carefully chosen second solvent enabled the not-quite-dry atomized droplets to form a smooth, continuous film of high quality over the OLED electrode. Because the films dry quickly on the surface, it should be easy to fabricate multilayer devices without mixing of materials between layers.

From a series of comparative experiments, the researchers found that devices fabricated from electrospray-deposited films turned on at lower voltages and could support higher current densities than ones made from spin-coated films. At low voltages, the electrospray deposition also enabled higher pixel intensity.

“We have discovered a range of conditions using a two-solvent method that can make extremely smooth thin films using electrospray deposition,” says Yamagata. “Using this technology these devices could be manufactured as inexpensively as printing newspapers.”

Yamagata also notes that: “The advantage of using electrospray deposition is that we can fabricate both smooth films and nanostructured film using the same technology.” In the future he believes that this advantage “will also be useful in controlling the structure of organic semiconductor junctions for organic solar cells.”